Stereoscopic motion pictures have been displayed by simultaneously projecting right and left stereo images upon a screen in overlapped relationship while transmitting the light of each image through mutually extinguishing filters, and viewing the overlapped images on the screen through similar mutually extinguishing filters. These filters may comprise colored filters such as red and green, or polarizing filters disposed with their planes of polarization normal to each other.
Prior art devices have used two projectors and two film strips to overlap the projected stereo images upon the screen, one projector having the film for right eye images and the other for left eye images. In the two projector systems, the images were difficult to synchronize, adjustment of the position of the overlapped images was time consuming, and the image light intensities were frequently out of balance. Such systems also necessitated the additional expenses of twice as much film as standard motion pictures, extra handling in the projection booth, synchronization, etc. As a result of these technical difficulties and extra costs, the two-projector stereoscopic process is seldom now used by the motion picture industry.
It has been proposed to print left and right motion picture stereo pictures on a single film frame and to project them simultaneously using a single projector. The stereo pictures may be printed side by side upon the film or one above the other. Mirrors have been employed to bring the two images into overlapping relationshp upon the screen. Such optical devices, however, have been responsible for substantial light loss resulting in dull, unsatisfactory projection. Moreover, prior art optical devices employed for stereo projection of single strip stereo images are bulky, difficult to adjust, and often require substantial modification for use with the standard motion picture projector.
Most of the above problems of the prior art were solved to a considerable extent by an adapter which is disclosed in U.S. Pat. No. 3,851,955. In accordance with this patent, the adapter comprises a housing which was secured to the projection lens of a standard motion picture projector. The projector was supplied with a motion picture film having a series of abutting stereo image pairs thereon: Light from the stereo pairs was directed through two prisms carried by the housing. The prisms were mounted and were angularly adjustable to enable the stereo pairs to be aligned in overlapping relationship upon a theater screen.
Light polarizing filters were incorporated with each of the prisms with their planes of polarization at 90.degree. with respect to each other. The prisms were disposed within the housing in close proximity to each other and abutting along one margin. To accommodate the prism assembly to various screen throw distances and screen sizes, the conventional projection lens was supplemented or replaced by a zoom lens, whereby focus may be re-established along with precise positioning of the overlapped imaged upon the screen. The film was printed with an opaque band between the stereo pairs of a single frame. When the pairs overlapped the opaque band formed sharp black edges for the image projected on the screen, such as conventional projection produces. The meeting edges of the prisms in front of the projector were also masked by the opaque band.
Although the above system solved a number of the disadvantages of prior art systems, it suffered from a number of inadequacies. For example, it required achromatic prisms to deflect the images. These prisms are expensive. They caused a slight linear and chromatic aberration of the projected image and were bulky and heavy. The primary prism has an angle of about 4.5.degree.. To increase this angle for certain projection distances the system may require the hand insertion of trimmer prisms having an angle of one or two degrees. These trimmer prisms, for reasons of economy, are not achromatic.
The invention disclosed in U.S. patent application Ser. No. 331,779 filed Dec. 17, 1981 addressed these problems. It provided for a conventional projector with optics for 3-D projection with improved economy, simplicity of installation, and a minimum of bulk known as Polarator II.TM.(A). The same was achieved by shortening the path length of the device by passing the light from the film in the projector through a prism reflector having index of refraction of 1.53 or more and twice deflecting each of the individual images of a 3-D image pair to be focused upon the projection screen in the desired position. In accordance with the preferred embodiment the same was achieved by using a pair of simple non-achromatic prisms with surfaces having reflective coatings. In accordance with this preferred embodiment, one prism of each of the pairs is provided with an angularly adjustment mounting to provide for the alignment function. No trimmer prisms are needed.
In a copending application Ser. No. 06/427,576 filed Sept. 29, 1982 improvement was sought through the elimination of the reflective coatings on the inner and outer prisms in the device, and the substitution therefor of total internal reflection. Total internal reflection is effected by an appropriate angular positioning of the prisms and by the type of glass used. This improvement, known as Polarator II.TM.(B), results in lower cost of manufacture and increases the light transmission efficiency by about 20%.
In the above-noted parent application a beam splitter device is disclosed which utilizes polarization by reflection and transmission to split a beam of incident light into two beams polarized mutually perpendicular to each other. These polarized beams are directed onto the images of a stereo pair occupying adjacent halves of the film gate area of a motion picture projector. Subsequently, dichroic polarizers were used to increase the percentage of polarization from about 95% after transmission of the light through the polarizing beam splitter to about 99.9% in the beam projected onto the screen. Since the dichroic polarizer transmits the parallel component of polarized light with an efficiency of 70-80% and since the polarizing beam splitter disclosed in the parent application had an efficiency of about 80%, the overall light efficiency of the device was 56-64%. In the prsent invention a crystal beam splitter is employed which has a 95% light transmittance; hence the overall system efficiency is increased to 76%; and when using the light funnel 37 on FIG. 7 herewith fully described in copending application Ser. No. 463,538 is about 100% of that of the conventional projector light output.